1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to cleaning devices for streamers used in marine seismic surveying and, more particularly, to autonomous cleaning devices and related methods for cleaning marine growth and other contaminants deposited on marine equipment towed underwater.
2. Discussion of the Background
Marine seismic surveying investigates and maps the structure and character of geological formations under a body of water using reflection seismology. Reflection seismology is a method of geophysical exploration especially helpful in the oil and gas industry. In marine reflection seismology, the depth and the horizontal location of features causing reflections of seismic waves are evaluated by measuring the time it takes for the seismic waves to travel to receivers. These features may be associated with subterranean hydrocarbon reservoirs.
A typical marine seismic surveying system is illustrated in FIG. 1. A vessel 100 tows a seismic source 102 and plural streamers 106 (only one shown for simplicity), with each streamer carrying an array of seismic receivers 104 (e.g., hydrophones, geophones, accelerometers, or a combination thereof). It is desirable to maintain the streamers at predetermined horizontal cross-line distances (i.e., along an axis perpendicular to the towing direction T), and at predetermined depths (e.g., 10 m) relative to the water surface 108. The seismic source 102 is configured to generate a seismic wave 110 that propagates downward (down, up and vertical being defined relative to gravity) toward the seafloor 120 and penetrates formations 125 under seafloor 120 until it is eventually reflected at discontinuous locations such as 122a and 122b. The reflected seismic waves 130a and 130b propagate upward and can be detected by one of receivers 104 on streamer 106. Based on the data collected by receivers 104, an image of the subsurface formation is generated by further analyses of the collected data.
To maintain the streamers at a desired position (i.e., such as to have predetermined cross-line distances and predetermined depths), conventionally, a head float 140 and a tail buoy 150 are attached to the streamer. Position control devices 152 (e.g., birds) may be attached to the streamer, every 300 m, to control the streamer's position.
Significant amounts of bio-fouling settlement accumulates on the streamers' exterior surface, which can obscure the reflected seismic wave and significantly increase streamer drag. The rate of accumulation and the impact of bio-fouling and other contaminants depend on factors, among others, such as geographic location, water temperature, and season. The gooseneck barnacle is the most common bio-fouling organism found on marine streamers.
Cleaning such contaminants from the streamers' exterior is desirable and beneficial. For example, a cleaning device 160 may be moved along the streamer to clean contaminants from the exterior thereof.
An active cleaning device, such as the one disclosed in U.S. patent application Ser. No. 14/156,818, assigned to the assignee of the present application, includes one or more cleaning elements (e.g., brushes) and a mechanism configured to attach and to roll the cleaning device along the streamer in order to clean its exterior. More specifically, as illustrated in FIG. 2, a cleaning device 200 is located around a streamer 202 and configured to autonomously, i.e., without human intervention, move up and down the streamer, between two obstacles. Cleaning device 200 has a body 210 configured to support one or more wings 212. A cleaning tool 220, e.g., a brush, a magnet, etc., may be attached to body 210. In one application, the cleaning tool may have an anti-fouling coating or may provide an anti-fouling coating to the streamer. One or more cleaning tools may be located on cleaning device 200. A switching and locking mechanism 214 is attached to body 210 and determines, when contacting stoppers 204 or 206, a change in the wings' orientation (e.g., angle 213). By changing the wings' orientation, cleaning device 200 may rotate, like a screw, about streamer 202, along one of the two directions 218 and 219. Depending on the orientation of wings 212, a cleaning device may rotate clockwise or counter-clockwise around streamer 202. This rotation determines how the cleaning device moves along the streamer in any of directions 230 and 232.
However, it is known that the streamer has various parts and/or components, some with a larger diameter than others, and an efficient cleaning device needs to be able to cross these parts while it cleans. Further, the cleaning device's existing mechanisms for changing wing position are complicated and prone to failure, especially if marine fouling gets inside these mechanisms.
Therefore, there is a need to develop streamer cleaning devices that operate, autonomously, upstream and downstream over variable diameter streamers, with reliable mechanical parts not prone to failure.